This invention relates to sealed batteries and a method for detecting overcharging thereof, more specifically to sealed batteries with a hydrogen gas oxidation auxiliary electrode capable of detecting overcharging for prevention thereof.
Heretofore, in charging sealed batteries such as nickel-zinc batteries, nickel-cadmium batteries and lead-acid batteries, plenty of hydrogen and oxygen gases have been produced at the negative and positive electrodes respectively in case of overcharging, thereby causing various problems including an increase of the battery pressure which would lead to expansion of the container or leakage of the electrolyte. While these problems may be solved by detecting overcharging and stopping charging, there have conventionally been proposed the following methods of overcharging detection.
(1) A method in which the battery terminal voltage is measured. PA0 (2) A method in which the quantity of electricity with which the battery is charged is measured by using a coulometer. PA0 (3) A method in which the temperature of the battery or a catalyst therein is measured. PA0 (4) A method in which the specific gravity of the electrolyte is measured.
However, these methods were not satisfactory for practical use because of their substantial errors in overcharging detection. In method (1) wherein the battery terminal voltage is measured to detect the terminal voltage rise at time of overcharging, thereby detecting the overcharging, for example, the terminal voltage was substantially affected by the changes of the internal resistance of the battery and the ambient temperature, so that it was difficult to secure high-accuracy detection of overcharging.
Further, in an alternative prior art method, overcharging is detected by measuring the current flowing between the negative electrode and an oxygen gas oxidation auxiliary electrode disposed within the battery. In this method production of the oxygen gas at overcharging may be detected by measuring the oxygen gas absorbing current flowing between the auxiliary electrode and the negative electrode. This method, however, is subject to a shortcoming that the auxiliary electrode may easily be oxidized and deteriorated by the oxygen gas, thereby reducing the sensitivity as well as the overcharging detecting accuracy.